(1) Field of the Invention
This invention relates generally to video signal processing and, more particularly, to digital processing of the chrominance signals that are developed in a color video system.
(2) Description of the Prior Art
Considerable attention has recently been given to the area of digital coding of color video signals, in general, and to the reduction of the number of bits needed for good picture rendition, in particular. One coding technique that has been suggested is described in U.S. Pat. No. 3,803,348, issued to J. O. Limb et al on Apr. 9, 1974, and assigned to the same assignee as the present application. The Limb technique is based upon the assumption that significant chrominance changes generally occur in the vicinity of significant luminance changes. Accordingly, Limb's apparatus is arranged to detect instances when the difference between adjacent luminance samples is found to exceed a preselected threshold level. When a significant change is detected, the chrominance signal that is transmitted is an average of the chrominance values between successive significant changes. On the other hand, when the luminance signal is relatively flat, no chrominance signal is transmitted, thereby reducing the number of bits that are needed.
For the few occasions when chrominance changes occur without corresponding luminance changes, a modified approach, as set out in U.S. Pat. No. 3,860,953, issued to C. C. Cutler et al on Jan. 14, 1975 may be used. In this technique, when a chrominance change occurs in a region in which no corresponding luminance difference is present, the luminance signal is intentionally modified to introduce therein a significant change. Accordingly, when the desired chrominance information is then transmitted, no addressing information is needed at the receiving decoder, which simply detects both the introduced and the regularly occurring luminance changes.
While the foregoing and other similar techniques attain the primary objective of decreasing the number of bits which must be utilized to transmit chrominance information, it has been found that the subjective quality of the pictures produced is often not satisfactory. Accordingly, it is the broad object of the present invention to improve image fidelity while maximizing coder efficiency.
The foregoing and additional objects are achieved in accordance with the invention by a technique which varies the degree of precision with which chrominance signals are encoded as a function of the spatial activity of the luminance signal in the neighborhood or region being processed. Specifically, it has been found that for neighborhoods of high luminance spatial activity, the chrominance signal may be coarsely encoded without noticeably sacrificing picture quality. Conversely, in areas of low luminance activity, the chrominance signal should be encoded more finely (with a higher degree of precision) since inaccuracies are more visible in such regions.
The above-described principles are generally achieved by (1) defining a measure of the luminance activity in a particular area of the image being processed and (2) changing the coding strategy of the corresponding chrominance signal in accordance with the activity measure.
In one specific arrangement, the activity measure is computed from a weighted average of luminance slopes of picture elements in the neighborhood being processed, and one of several quantizers is inserted in the chrominance coder depending upon the value of the average. In another arrangement, the activity measure is computed as the maximum weighted slope in a given neighborhood, and quantizer precision is adjusted by simultaneously varying the gain of an amplifier disposed in the forward loop of the chrominance quantizer and the attenuation of an attenuator disposed in the feedback loop. As a still further alternative, coding strategy of one chrominance signal is based jointly on luminance activity and the spatial activity of the other chrominance signal.
By processing chrominance signals in accordance with the present invention, it has been found that for a given picture quality, the number of bits needed for quantization is reduced by 20-30 percent as compared to prior art arrangements. Alternatively, if the bit rate is not changed, the present invention achieves significantly better subjective picture quality compared to conventional encoders.